Bandwidth of neon xfmr

For frequencies below 60 Hz: Input voltage must be reduced proportionately with frequency once you go only a few percent below 60 Hz. Output short circuit current should not change much in such a case but will decrease more significantly once the frequency gets below about 20-30 Hz or so, and will decrease nearly proportionately with input voltage (proportionate with frequency) once the frequency is low enough for output current limitation to be due mostly to winding resistance rather than "leakage inductance" - and I guesstimate this point to be roughly 15 Hz or somewhere in that ballpark. But more importantly I suspect that when using a transformer recommended for a specific length or set of tubing, 1/3 to 1/2 voltage may be insufficient to spark through at all - which means a low frequency limit of around 20-30 Hz to get any results at all unless the input vopltage is kore than the transformer should get at the frequency in question.

As for frequencies higher than the transformer is designed for:

As frequency increases, there is a trend for the maximum permissible input voltage to increase, but less than proportionately since one significant loss mechanism producing heat is eddy current loss, which does not decrease much when frequency deviates significantly upward from "proper" and "proper" input voltage is maintained. But do expect the "leakage inductance" to cause output current to decrease as frequency increases!

Operate a neon sign transformer rated for 60 Hz at 400 Hz, and I would guesstimate it would have its usual temperature rise when the input RMS voltage is about 150-160 volts. Output current to a neon tubing load at

400 Hz instead of 60 with normal input voltage I guesstimate to be about 16-20% of "normal", and somewhat more than proportionately more (possibly 30% of "normal") with input voltage of 160 volts.

Open circuit output voltage at 400 Hz: With 120V input, most likely slightly higher than normal, but possibly very different and even possibly much more. Please have a load connected when operating a neon sign transformer at frequency much above rated!

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Neon sign transformers have a resonance mode at some higher frequency, where the "leakage inductance" and the stray/interlayer capacitance of the secondary form a series resonant circuit. In my limited experience with a small sample size this has happened at a frequency around 1-1.5 KHz as best as I can remember. Such a resonance mode can be a serious problem with no load - output voltage can be a few times that otherwise expected, and can be a major stress on or even causing outright quick failure of the insulation within the transformer! Please have your "neon" tubing connected!

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Once frequency increases to much above that of the "resonance mode", expect open circuit output voltage to be a little to somewhat more than "expected" times the square of the ratio of "resonant frequency" to applied frequency. As in mostly below usual and decreasing bigtime as frequency increases. Additional "resonance modes" may at some frequencies cause some significant deviations from this, but expect mostly output voltage awfully low once the frequency exceeds a few KHz. As for output current - mostly keeps going down as frequency increases. (Exceptions possible but not the "norm".)

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This is for the "traditional" iron core transformers as opposed to the newer "electronic ballast" type devices. The latter should have performance varying less with input frequency and hardly changing at all when input frequency makes a moderate change, although probably have nowhere to go but down in any performance perameter (including reliability) if the input frequency is changed greatly.

- Don Klipstein ( snipped-for-privacy@misty.com, snipped-for-privacy@netaxs.com)

Reply to
Don Klipstein
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I know neon transformers are designed to operate from 60Hz. But what range of input frequencies can they handle and still produce enough juice to light a small neon tube?

Kev

Reply to
Kevin Tate

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